Cylindrical dryer



CYLINDRICAL DRYER 3 Sheets-Sheet 1 Filed Sept. 9, 1963 INVENTOR.

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CYLINDRICAL DRYER Filed Sept. 9, 1963 3 Sheets-Sheet 2:

INVENTOR.

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CYLINDRICAL DRYER Filed Sept. 9, 1965 3 Sheets-Sheet 3 INVENTOR. fizz gar cl z/(Is'zw Kobe/a 19 Daafle TORNE YS United States Patent 3,241,251 CYLINDRICAL DRYER Edgar J. Justus, Beloit, Wis., and Robert A. Daane, Rockford, Ill., assignors to Beloit Corporation, Beloit, Wis., a corporation of Wisconsin Filed Sept. 9, 1963, Ser. No. 307,578 7 Claims. (Cl. 34124) This is a continuation-in-part of our copending application Serial No. 7426, filed February 8, 1960, and now abandoned.

This invention relates to a shell for use in the rotatable drum type of unfired pressure vessel of the type used in paper-making machinery as dryers, gloss calender rolls, hot press rolls, extensible paper units, etc. The shell member of this invention may form a part of such assemblies which are otherwise known in the art.

Although the instant invention relates to any of the aforementioned uses and vessels involved in such uses, it will be noted that the invention has particular utility in connection with improved paper machine dryer structures, and the invention will be described in connection therewith. Such dryers are characterized by a smooth, regular, continuous outer cylindrical surface over which a web of paper, or the like, is run continuously to be smoothed, dried, or glossed by the application of heat, and in some instances pressure. In the case of the conventional paper machine dryer, however, only nominal pressure is usually employed, but in the case of the so-called Yankee dryer, which is really a giant-sized dryer drum, rolls are used to define a nominally loaded press nip with the Yankee dryer. It is conventional in the case of each of such dryer rolls to heat the same by the admission of steam under controlled pressure into an interior chamber defined by the roll shell and to transfer the heat of condensation through the shell to the web being treated.

The instant invention has particular utility when the shell, which will be described in detail hereinafter, forms a part of a dryer or roll used in conjunction with another roll to form a press couple having a nip which constitutes a load line for the application of pressure to the web. As previously mentioned, the instant invention may also be utilized with particular success in the case of the conventional dryer shell that is not subject to such a load line.

Many proposals have been made for making use of internal projections such as ribs for the purpose of reaching through the layer of condensate formed on the inner wall of a dryer so as to expose the metal of the shell to direct contact with the condensing steam. Typical of these are Hutchins US. Patent No. 1,453,113 and Hornbostel US. Patent No. 2,521,371. Hornbostel, however, teaches triangularly shaped rib sections, which we have found to be undesirable. Tensile stresses, circumferential, at the inner radius of such ribs require greater shell thicknesses. Longitudinal stresses also are concentrated by the relatively sharp V-shaped groove. Increased shell thickness not only reduces heat transfer but also, since the shell temperature differential (inside to outside) is greater, results in temperature stresses that are higher.

To the best of my knowledge and belief none of the dryer shells, of which the aforesaid patents are typical, has ever been put into practical use. The drying of paper and the like materials in web form requires that there be nearly perfect uniformity of heat transfer rate through each unit of the surface area from the heating fluid to the web. Heretofore, those skilled in the art have summarily rejected the use of grooving projections, and the like because of the belief amounting to certainty that neither surface temperatures nor heat transfer rate could possibly be uniform through such a non-uniform wall.

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The present invention resolves these problems by providing a dryer drum or similar shell having a relatively thin shell for effective transmission of heat from the inner diameter to the outer diameter of the shell or drum; and such shell is formed with inner circumferential axially spaced ribs which actually strengthen the drum or shell and permit the safe use of a significantly thinner shell through which the transfer of heat is improved. The generally rectangular configuration of such ribs avoids the risks of local failure and further exposes significantly greater area to the heating fluid. The troughs defined between the ribs are provided in a configuration such that steam may come into contact with the maximum diameter portions of the inner drum surface, while being as narrow as is consistent with removal of water from the troughs and with ease of manufacture. Desirably, the width of the ribs is never more than about three times the groove width. With wider grooves, the rib width to be preferred more nearly equals the groove Width, and because of the described strengthening effect of the ribs, a dryer drum having significantly improved drying capacity is provided at no sacrifice of strength as compared to conventional constructions.

Accordingly, it is an object of the present invention to provide an improved dryer drum which is capable of sustaining loads equivalent to the loads which may be imposed on conventional drums of the same size, but which affords greatly improved heat transfer from the inner surface to the outer surface thereof.

Another object of the invention is to provide a drum as described having a rib-reinforced thin shell with steam receiving troughs between the ribs which are kept free from water so that the steam will be in contact with the shell as close as possible to the outer diameter of the shell.

Another object of the invention is to provide a dryer drum as described which eliminates concentration of stresses at the inner surfaces thereof such as has occurred with previous constructions having pointed ribs, whereby the tendency toward local failure inherent in such constructions is eliminated. 1

Another object of the invention is to provide a drum as described having relatively deep steam receiving grooves in close heat transfer relationship to the outside of the shell, and dipper means for removing condensate from the grooves without loss of strength or uniformity in heat distribution.

Yet another object of the invention is to provide a device as described wherein the grooves have a relatively small Width and are constructed in predetermined ratio to the width of the ribs such that the heat transfer to the outer surfaces of the drum is substantially uniform.

Other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed disclosure thereof in the drawings attached hereto and made a part hereof.

On the drawings:

FIGURE 1 is a vertical section of a dryer drum according to the'present invention (showing the shell schematically, although it is shown in detail in the portion encircled and designated V, in FIGURE 5);

FIGURE 1A is an essentially diagrammatic view showing a pair of heated drums in nip-defining relationship for treating a paper web;

FIGURE 2 is a vertical sectional view taken along the lines II--II of FIGURE 1;

FIGURE 3 is an enlarged fragmentary vertical sectional (non-scale) view of the circumferential wall of a drum according to the invention, although not drawn to scale;

FIGURE 4 is a fragmentary vertical sectional view of the structure shown in FIGURE 3 in operative relation to dipper means for removing condensate from the grooves of the invention;

FIGURE 5 is a view (taken from the encircled portion marked V in FIGURE 1) comparable to FIGURE 3 but shown in full scale with dimensions indicated, and in full view for ease of indication of dimensions; and

FIGURE 6 is a chart plotting generally the shell diameter in feet against the dimension A in inches (which is the dimension A indicated in FIGURE 5 as the shell thickness at the bottoms of the troughs or grooves), wherein the area X indicates the dimension A preferred for use with various size dryer shells.

As shown on the drawings:

Referring now to the drawings, a dryer drum 10 is shown according to the present invention having a cylindrical shell 12 which is closed by end heads 14 and 16 which may be bolted to the shell as indicated at 17. The end heads 14 and 16 have journals 18 and 20 connected thereto by suitable plates 22 and 24 which may be secured in annular recesses 26 and 28 in the end heads, and the journals 18 and 20 have respective conduit portions 30 and 32 extending into the shell from the end heads and connected in a conventional manner by means of bolting flanges 34a and 34b. Communication between the conduit portions 30 and 32 is prevented by a blanking plate 340 which also provides for precise fit-up of the journal to head assembly. Steam nozzles 36 and 37 may extend from openings 39 and 40 in the plate 340 for introducing steam centrally into the interior of the drum as transmitted through one of the journals such as the journal 18 as understood by those skilled in the art. In order to remove condensate from the interior of the drum, dipper means are provided as hereinafter further described and which may include dipper pipes 41 and 42 extending longitudinally for substantially the length of the drum and maintained in proximate relation to the interior peripheral wall of the shell 12 by radially extending rods 43 and 44 connected at their inner ends to the flange 34b and connected at their outer ends to the respective dipper tubes 41 and 42 by brackets 46 and 48. The brackets 46 and 48 may include gussets as indicated at 50 and 52, and are secured to the rods 43 and 44 by rod ties 54 and 56 and hex nuts 58 and 60, by which precise radial adjustment may be secured. As seen more particularly in FIGURE 2, the dipper tubes 41 and 42 communicate with the interior of the journal structure 32 by means of syphon pipes 62 and 64 which may extend into oppositely disposed and relatively slightly offset bosses 66 and 68 in the said structure 32.

The pipes or dipper tubes 41 and 42 are held in position at their outer ends by plate tension means 70 and 72 and 74 and 76 each secured to respective bracket elements '78 and 80 and 82 and 84 on the respective heads 14 and 16, and also connected to brackets 86 and 88 and 90 and 92 on the tubes 41 and 42 respectively. The element 82 is shown as connected to a counterweight 94, although other constructions may be utilized within the scope of the invention.

The end heads 14 and 16 may also be provided with manholes 96 and 98 with covers 100 and 102, and a petcock 104 for the end head 16.

In accordance with the invention, the inner diameter of the shell 12 is formed with a plurality of circumferentially continuous and axially spaced ribs as indicated in FIGURE 3 by reference numeral 106, and the ribs 106 define relatively deep and narrow troughs or grooves therebetween as indicated at 108. In the example shown in FIGURE 3, the ribs 106 and the troughs 108 are of shown schematically substantially equal and uniform width. The inner end surfaces 110 of the ribs are preferably straight in axial cross section and are formed with the sides 112 and 114 thereof extending radially and in perpendicular relationship to the axis of the drum. The ribs 106 have a width of /8" and the troughs 108 have a width of /2, for example, with the bottoms 116 of the grooves preferably curving toward the sides thereof with a fillet radius of and the top edges of the ribs 117 and 118 having a radius of In the preferred embodiment the ratio of rib width to groove width is thus 5:4. The grooves may be exceptionally deep, and thus permit steam to come into intimate contact with a large area of the surface of the drum which is disposed in proximate relationship to the outer surface thereof. Thus, a radial dimension for the ribs of 1 /8", in a cast iron shell having a minimum thickness of 1%", affords a shell having resistance to circumferential nip load stress, pressure stress and thermal stress which compares favorably with a cast iron shell of uniform thickness of 1% and similar size. At the same time, however, a 30% improvement in heat transfer is afforded, while thermal uniformity on the outside surface is preserved.

It may be noted that longitudinal stresses are not significant, in comparison with circumferential stresses, except near the ends of the drum, so that a standard end construction is satisfactory. However, to avoid relatively large longitudinal stress effects in the grooved portions of the shell, the distance of these portions should be in the neighborhood of 6" from the ends.

Referring briefly to FIGURE 1A, it will be seen that two such rolls 210 and 210a may be used to define therebetween a nip N through which guide rolls 211 and 211a may guide a paper web W. The nip is optionally pressure loaded and the rolls indicated diagrammatically at 210 and 210a are heated by the introduction of steam into the interiors thereof and these rolls 210 and 210a have generally the shell construction hereinbefore described. The paper web W thus passed through the nip N is hot pressed in such nip N and there is, of course, a load line defined at the nip N for the application of pressure to the web W.

In order to permit the removal of the condensate from the grooves, as shown with respect to the embodiment of FIGURES 1 through 4, each of the dipper tubes 41 and 42 has a plurality of radially extending tubes 128 spaced axially therealong and secured by tack welding or other wise at their ends to be firmly bottomed on the shell and in position to open in proximate relation to the bottoms of the grooves 108, such as to afford removal of the condensate through the tubes 62 and 64 and into the conduit portion 32 for outlet through a tube 130 in the journal 20 corresponding to the inlet tube 132 in the journal 18. In the form shown, the ends of the tubes 128 are beveled at 134 so that the tips of the tubes rest on the bottoms of the grooves 108, but other constructions may be utilized such as forming a shoulder on the tubes to rest on the ribs.

Although the ribs of the invention have been described as generally rectangular in cross section and generally circumferential or annular in construction, a gradual helical configuration may be utilized, if it is suitable for a particular situation, in which case the angle of the helix is desirably 10 or less. In the case of such helical grooves, condensate removal systems generally arranged to rotate with the dryer assembly, as shown herein, are used. In the conventional Yankee dryer, the center shaft requires this. With dryers not employing a center shaft, however, a stationary or non-rotating condensate removal system may be used, but in such instance there is a distinct disadvantage to the use of helical grooves because the individual tubes of the type here shown may not be used on a stationary mounting with helical grooves for evacuating the grooves individually.

The ribs are continuous in order to effect the desired stiffening or rein-forcing effect. The rib section is preferably generally rectangular, as previously indicated, and may be produced by machining, turning with single point, gang-tools or mill cutters, or by milling with a planetary head. The latter could, of course, produce a relatively trapezoidal rib, but it should be borne in mind that it is desired to keep the moment of inertia of the rib at a maximum, other dimensions being equal. The triangular type of rib section, as previously discussed, is definitely not desired. This latter configuration permits stress concentration at the section apex and offers considerably reduced corrosion life as compared with the rectangular section.

Experience, statutory requirements in some states, and various codes require that cast iron vessels of this general type be limited to 160 pounds per square inch (gauge) maximum steam pressure in order to keep the indicated maximum safe unit stresses within the predictable strength of the cast iron used. It has been found in the present structure that it is possible to obtain the desired reinforcing effect and also to obtain a practicably perfect uniformity of heat transfer rate throughout the working surface area of the shell. It has also been found that this dryer construction can deliver as much as 30% more heat units into the web being dried or treated as contrasted to prior art dryers of comparable dimensions and steam pressure rating. Moreover, calculation and test results now indicate that the line loads applied to such dryer shells by mating press couple members (e.g. FIG- URE 1A) may be at least as great as with a conventional dryer shell or can even be increased without exceeding acceptable factors of safety.

In the manufacture of paper and paper products, numerous grades, namely, sanitary tissues, crepe wadding, glazed papers, uncreped extensible paper, certain coded papers, and the like require that the sheet or web be adhered to or firmly pressed against a smoothed surfaced drying cylinder by a roll in line contact. In such a pressing couple, the nip pressure (usually quoted in pounds per lineal inch along the line of contact) may range from a relatively nominal figure of 0 to 25 pounds per lineal inch to very high loads in the neighborhood of 400 pounds per lineal inch or more. It has been noted that the heat transfer effect, i.e. the drying rate, and the smoothing effect are both markedly improved at a higher nip pressure. Such line pressures, heretofore, have required a compromise on heat transfer rate since it has beennecessary to use dryer shells of increased thickness in order to provide the necessary stiffness and strength to bear the nip loads desired. As a result of our work we have found that the rib structure disclosed not only provides marked advantages in heat transfer rate but also the extra utility of reinforcement against such line loads, all without sacrificing the desired uniformity of heat transfer rate.

It will be appreciated that the radial dimensions of the ribs in each instance are such as to resist the circumferential stresses, whereas the thickness of the shell between the ribs is determined in accordance with the expected longitudinal stresses. Although various grooveto-rib ratios have been indicated, it has been found that this ratio should not be below about 1:3.

Both nip loading and pressure stresses, in general, call for a maximum shell thickness for resisting circumferential stresses which is approximately twice the minimum thickness for resisting stresses in the longitudinal direction. With the groove-to-rib ratios here employed, thermal stresses do not exceed those of conventional dryers, and since the thinner wall afforded by the invention produces greater heat transfer, a smaller temperature difference is encountered from the inside to the outside of the shell, with consequent improvement in the drying capacity of the shell. Because of the configuration of the ribs of the invention, there is no significant concentration of local stresses at the ribs, and the bending moment is of relatively small effect with the possible exception of the areas immediately adjacent the drum ends, as previously indicated.

In general, it is desired that the stress-.to-strength ratios in the three coordinate directions, at a point on the drum, be nearly equal and that the circumferential and axial stress-to-strength ratios approach equality, the con- 6 struction herein set forth effectively producing this result.

It may also be noted that the heat transfer surface on the interior surface of the drum is large, since the radial surfaces of the ribs are kept substantially clear of condensate .by centrifugal forces; and the positioning of the tubes in the grooves removes the condensate at the groove bottoms as described. Accordingly, a dryer drum has been provided which effectively resists the stresses therein, particularly nip load stresses, and which presents no danger of local failure, such as had been produced in various drums heretofore as a result of successive slight expansion and contraction during heating and cooling. The instant drum is at least as strong as con-ventional drums of substantially the same size, and it can be used in a wide variety of applications to deliver a substantially greater number of units of heat.

Certain parameters of the invention have been established by research and experimentation and these may be described conveniently in connection with. FIGURES 5 :and 6. It Will be noted first that in FIGURE 5 (which in contrast to FIGURES 3 land 4 is representative of a drawing to scale) the shell thickness at the groove 208 is indicated by the dimension A, whereas the shell thickness at the rib 206 is indicated by the dimension B, and the radial dimension of the rib 206 is indicated as being 1%" in this preferred embodiment. The rib thickness, indicated by the dimension D is here shown as in the preferred embodiment; and the corresponding groove width indicated by the dimension E is /2" in this preferred embodiment. The pitch or sum of the groove Width E and the rib width D is indicated by the dimension C.

In the practical design of a pressure vessel or shell according to the instant invention, the shell thickness, dimension A, should be selected in accordance with the requirements of safe longitudinal unit stress. This stress is, of course, a function of the longitudinal pressure load and is dependent on internal steam pressure, shell diameter, and nip load sheer stress. The values for A are thus determined conventionally, but for convenience FIG- URE 6 here designates in area X the value ranges for A in inches using various shell diameters. As typical examples, for a 12-foot diameter dryer, the dimension A should preferably be between /8 and 1 /2"; whereas for a 10-foot diameter dryer, the dimension A should be chosen between about /2 and 1.2"; and for a 5-foot diameter shell, the dimension A should be between about A" and 1" (as is more clearly demonstrated as an inherent feature in the scale drawing of FIGURE 5). The foregoing figures apply to high-grade cast iron such as is conventionally used for Yankee dryers. In Yankee dryers, an additional consideration may require somewhat greater thicknesses (as the skilled worker in this field knows), depending not only upon the integrity of the material in cast form but also upon the service for the dryer, to the extent that refinishing of the surface may be required more or less frequently in order to maintain the required finish for such service. The dimension limits preferably indicate the final size after regrinding, and it will be appreciated that certain safty factors and other considerations may require the use of dimensions A somewhat greater than are outlined on the chart of FIGURE 6. In FIGURE 5 the actual dimension A is 1%" and the dimension B is thus 2 /2.

It will thus be seen that, for FIGURE 5, the following relationships are obtained (shown generally to scale, in as-cast condition, which obviously may result in nominal dimensional changes which for practical purposes do not constitute a departure from the scale of FIGURE 5 here shown:

C:A equals 9:11 or 0.8 3 D:C equals 5:9 or 0.55 B:A equals 20:11 or 1.8 D:E equals 5:4 or 1.25

After the dimension A is determined, the pitch should be determined and, as previously indicated, the pitch C is the sum of the axial dimensions of one of the grooves E plus the adjacent ridge D. The relationship C:A is a ratio which should lie between about 2 and about 0.8. When this ratio exceeds about 2 non-uniformities of heat transfer rate tendto make the use of the dryer troublesome or even impractical, because the web to be treated or dried does not receive a level profile of heat input across its width. This permits non-uniformities in moisture content and/or surface quality in the finished prodnot. With ratios below about 0.8, the cost of manufacture is, of course, increased without any significant advantage. Furthermore increasing difficulties in removal of condensate will tend to make the heat transfer rate drop off toward the value for the comparable thick shell. Condensate removal, with reasonably effective performance leaves only a small layer of water. This is less restrictive to heat flow than the metal of the prior art thick shells. Ratios of C:A above 2 will promote heat transfer but the shell then tends to behave as would an unreinforced thin shell. In presently preferred designs, the ratio C :A is preferably within the lower portion of this range, i.e. about 0.8 to about 1.2.

Another important aspect of the instant structure may be expressed in terms of the ratio of rib thickness D to pitch C. It has been found that this should lie between about /2 and Preferably, it is about 0.55, as indicated, in that the rib width D is somewhat greater than the groove width E, in the approximate ratio of D:E equal to about 5:4.

Also, the effective thickness of the rib, as expressed by the ratio of B:A should lie between about 1.5 and 5. In the preferred dryer structure, however, the ratio B:A should be in the lower portion of this range, i.e. about 1.5 to about 2. Higher ratios may be employed in instances of high nip loads and the like.

As previously indicated the ratio D:E is preferably within the range of 5:4 to 3:1. It will be appreciated that by using a high ratio for D:E and/or a high ratio for B:A, one may obtain greater strength in the ribs than is absolutely necessary for the required stiffening effect, and in such instance it might be possible to provide an occasional axially aligned conduit interconnecting grooves in staggered relation or in such limited numhers that the overall stiffening effect of the ribs would not be effected.

It will be appreciated that although in the preferred embodiment the rib dimension D is somewhat greater than the groove dimension E, in certain instances the ration D:E may be substantially 1:1, particularly in those instances in which the ratio of B:A is 2 or less.

Since grinding in many instances may involve removal of as much as 0.030 to 0.040 inch to refinish a dryer surface, it will be appreciated that the dimension A as set forth in FIGURE 6 may be increased as much as inch (above the values in Area X) in a new shell to allow for regrinding and/or refinishing during use in certain types of service.

We claim as our invention:

1. A generally cylindrical dryer shell of heat-conductive structural material, adapted for mounting with heads sealing the ends thereof and carrying generally co-axial shafts for rotation of the shell about its centroidal axis, said shell having a generally smooth outer peripheral surface portion and a plurality of alternating generally circumferential grooves and ridges on the interior periphery thereof, the radial thickness of said shell at said grooves being a dimension A, which is a dimension in inches selected substantially from the range A inch to 1 /2 inches the ratio of the sum of the axial dimensions in inches of one of said grooves plus an adjacent ridge to A being within the range of 0.8 to 2, the ratio of the radial dimension of said shell at a ridge to A being 1.5 to 5, and the ration of the axial dimension of a ridge to that of an adjacent groove being 5:4 to 3:1.

2, A generally cylindrical dryer shell of heat-conductive structural material, adapted for mounting with heads sealing the ends thereof and carrying generally co-axial shafts for rotation of the shell about its centroidal axis, said shell having a generally smooth outer peripheral surface portion and a plurality of alternating generally circumferential grooves and ridges on the interior periphery thereof, the radial thickness of said shell at said grooves being substantially 1% inches, the radial thickness of the shell at said. ridges being substantially 2% inches, the axial dimension of each of said grooves being substantially /2 inch, and the axial dimension of each of said ridges being substantially inch.

3. In a dryer for evaporating water from a traveling paper web, the combination comprising, a dryer drum having a hollow shell with a cylindrical outer surface, head assemblies at the ends of said shell bearing means for rotatably supporting the drum for rotation, means for directing a supply of steam into said drum, thereby creating a stress inducing pressure differential between the inside and outside of the shell, a plurality of individual, endless, annular, radially inwardly extending ribs within said shell integral with the shell and having side walls defining grooves therebetween, each rib being circumferentially continuous for uniform strength and deflection with rotation and progressive deflection due to said pressure differential, said ribs having an axial width no wider than three times the width of said grooves and no less than 5/ 4 of the groove width so as to be that required to sustain radial stresses induced by said pressure differential, and a condensate removal means projecting into each groove.

4. In combination, a dryer drum having a hollow shell with a cylindrical outer surface, head assemblies at the ends of said shell having bearing means for rotatably supporting the drum, nip loading means in stress inducing relationship with said shell, a plurality of individual, endless, annular, radially inwardly extending ribs within the shell integral with the shell and having side walls defining grooves therebetween, each rib being circumferentially continuous for uniform strength and deflection with rotation and progressive deflection due to said nip loading means, said ribs having an axial width no wider than three times the width of said grooves and no less than 5/4 of the groove width so as to be that required to sustain radial stresses induced by said nip loading means, means for directing a supply of steam into said drum, and a condensate removal means projecting into each groove.

5. A dryer drum comprising a cylindrical shell, a plurality of longitudinally spaced, circumferentially continuous ribs formed integrally within the shell and a plurality of circumferentially continuous grooves defined by said ribs, each of said ribs having side walls extending radially inwardly from said shell and in substantially perpendicular relationship thereto for a predetermined distance, said ribs defining a maximum shell thickness of substantially twice the minimum shell thickness at said grooves with the maximum shell thickness affording circumferential stress-to-strength ratio approximating the axial stress-to-strength ratio afforded by the minimum shell thickness and said ribs having a width between the perpendicular portions of said sides of from 5/ 4 to substantially three times the width of said grooves at the portions thereof defined by said perpendicular portions of said sides.

6. In combination, a generally cylindrical dryer shell of heat-conductive structural material, adapted for mounting with heads sealing the ends thereof and carrying generally co-axial shafts for rotation of the shell about its centroidal axis, said shell having a generally smooth outer peripheral surface portion and a plurality of alternating generally circumferential grooves and ridges on the interior periphery thereof, the radial thickness of said shell at said grooves being substantially 1 /8 inches, the radial thickness of the shell at said ridges being substantially 2 /2 inches, the axial dimension of each of said grooves being substantially /2 inch, and the axial dimension of each of said ridges being substantially /8 inch; and a condensate removal means projecting into each groove.

7. In combination, a generally cylindrical dryer shell of heat-conductive structural material, adapted for mounting with heads sealing the ends thereof and carrying generally co-axial shafts for rotation of the shell about its centroidal axis, said shell, having a generally smooth outer peripheral surface portion and plurality of alternating generally circumferential grooves and ridges on the interior periphery thereof, the radial thickness of said shell at said grooves being substantially 1% inches, the radial thickness of the shell at said ridges being substantially 2 /2 inches, the axial dimension of each of said grooves being substantially /2 inch, and the axial dimension of each of said ridges being substantially inch;

10 means for directing steam into the shell; and a condensate removal means projecting into each groove.

References Cited by the Examiner UNITED STATES PATENTS 1,453,113 4/1923 Hutchins 34-125 2,170,405 8/ 1939 Greenwood. 2,420,824 5/1947 Hornbostel et al. 34-125 2,521,371 9/1950 Hornbostel et al. 34-125 2,677,898 5/1954 Ohlson et al. 34124 2,817,908 12/1957 Hornbostel 34110 2,869,248 1/1959 Justus 34-125 FOREIGN PATENTS 497,034 5/ 1930 Germany.

ROBERT A. OLEARY, Primary Examiner.

NORMAN YUDKOFF, Examiner. 

2. A GENERALLY CYLINDRICAL DRYER SHELL OF HEAT-CONDUCTIVE STRUCTURAL MATERIAL, ADAPTED FOR MOUNTING WITH HEADS SEALING THE ENDS THEREOF AND CARRYING GENERALLY CO-AXIAL SHAFTS FOR ROTATION OF THE SHELL ABOUT ITS CENTROIDAL AXIS, SAID SHELL HAVING A GENERALLY SMOOTH OUTER PERIPHERAL SURFACE PORTION AND A PLURALITY OF ALTERNATING GENERALLY CIRCUMFERENTIAL GROOVES AND RIDGES ON THE INTERIOR PERIPHERY THEREOF, THE RADIAL THICKNESS OF SAID SHELL AT SAID 